In various processes in which a metal strip is moved along a predetermined moving path thereof it is extremely necessary to prevent deformation of and vibration of the moving metal strip. In addition, if the metal strip is deformed, it is very necessary to reform the deformed moving metal strip while the metal strip moves along its moving path.
For example, in a continuous hot galvanizing process for a steel strip, after the hot dip galvanized steel strip is subjected to a gas-wiping procedure for controlling the weight of the resultant coating, the wiped steel strip must move over a long distance between a pair of guide rolls without touching any other solid supports. Also, in a paint-coating process or in a heat-treating furnace for a steel strip, the steel strip must be supported, while moving over a long distance, between a pair of fixed guide rolls without contacting any other solid supports.
In the above-mentioned cases, the metal strip is sometimes vibrated and/or laterally warped so that it exhibits a C-shaped cross-sectional profile. This type of deformation of the metal strip is hereinafter referred to as "C-warping".
In order to support the metal strip moving along its moving path without using any solid supporting means, and at the same time prevent undesirable vibration and/or deformation of the metal strip, it has been attempted to utilize a fluid-cushioning effect derived from the application of a dynamic gas pressure or a static gas pressure onto both surfaces of the metal strip. In this type of supporting apparatus, it is important that the supporting apparatus stably support the metal strip and that the fluid used for supporting the metal strip not deteriorate the quality of the metal strip. For example, in a continuous hot galvanizing apparatus in which a gas-wiping procedure is applied to a galvanized metal strip, the metal strip is vertically removed from a galvainzing bath and is supported by a fluid stream ejected through a nozzle, formed parallel to the path along which the metal strip moves, without touching the fixed solid supporting means. In this case, the metal strip is locally cooled when the ejected fluid is cool or unevenly overheated when the ejected fluid is hot. This local cooling or heating causes the distribution of temperature on the metal strip to be uneven in the lateral direction of the metal strip. This phenomenon results in an unevenness in the galvanization of the metal strip, rendering the resultant product useless.
In another example, when a coated steel strip is dried while being supported by supporting fluid streams without touching the solid supporting means, the temperature distribution of the dried steel strip becomes uneven. This phenomenon causes the appearance of the resultant product to be deteriorated.
In still another example, when a metal strip is supported in a heat-treating furnace without touching the solid supporting means, the supporting fluid streams cause the temperature distribution on the metal strip to be uneven in the lateral direction of the metal strip. This uneven temperature distribution causes the quality of the resultant product to be uneven.
In a conventional supporting apparatus, it is considered important only to stably support the metal strip. That is, it is not considered how the supporting procedure affects the appearance and properties of the resultant product.